This post was updated at 6:15 p.m. ET
Readiness to be president is a threshold question for many candidates. That's especially true when that candidate is 43 years old and a freshman senator.
No, not Barack Obama, but Marco Rubio, the Republican senator from Florida, who announced Monday that he's running for president.
"I'm certainly capable from Day 1," Rubio told NPR's Steve Inskeep in an interview in Miami hours before he announced. "I'm very confident that I have the capability from Day No. 1 to lead this country."
Obama, for the record, was just a couple of years older than Rubio when he launched his bid for the White House in 2007. He was 45. Obama's election cuts both ways for Rubio. Some rank-and-file Republicans look at Obama and say the country needs someone with more experience. Others say if Obama could do it, then why not Rubio?
Still, Rubio's age and experience could be vulnerabilities in this presidential campaign, considering he is likely to be running against more experienced governors like his onetime mentor, Jeb Bush, 62, the former Florida governor.
Just two other Republicans have officially declared — freshman Sens. Ted Cruz, 44, of Texas and Rand Paul, 52, of Kentucky. Like Rubio, both were swept into the Senate with Tea Party support — Paul and Rubio in 2010, Cruz in 2012.
In the NPR interview, Rubio cited his experience at all levels of government as reason he is qualified to be president — serving as West Miami commissioner; as a member of the Florida state House, where he became speaker, and rising to the U.S. Senate.
"Given both my experience at the local, state and federal level, and especially the lessons learned over the last five years, as I've been engaged in federal policymaking, I'm very confident that I have the judgment and the knowledge that I need to do a very good job as president of the United States," Rubio told Inskeep. "Clearly, if you see the history of the presidency, people that go into that office grow. The office helps you grow as well as you go through it. Having the ability to learn is a big part of the office as well."
Rubio noted, however, that he, like any serious potential candidate, did reflect on his readiness.
"I think anyone who's thinking about highest office in the most powerful nation on Earth has to spend some time analyzing whether they're prepared for the job," he said. "And look, I think the job is perhaps the most difficult job in politics anywhere in the world. You're the leader of the most powerful military on the planet; you make decisions that have global implications, and it's happening at a time when the country is going through an extraordinary economic transformation that is leaving many people insecure."
But any second thoughts weren't coming on the eve of big announcement.
"Not last night," he said. "If you've reached the point that you're wondering it the night before you announce, then you probably shouldn't run."